Abstract Proper B cell tolerance ensures effective censoring of autoreactive B cells thereby avoiding their participation in effector pathogenic response and preventing autoimmune diseases. The breakdown of B cell tolerance is at the core Systemic Lupus Erythematosus and may in fact, represent the initiating step in the cascade of pathogenic events leading to clinical disease. While B cell tolerance is relatively well understood in transgenic mouse models, human B cell tolerance remains poorly understood due to the lack of informative systems and inherent experimental limitations. To remedy these problems, we have concentrated over the last few years in the study of autoreactive 9G4 B cells, which represent an abundant and homogeneous population of autoreactive B cells that are strictly censored in healthy subjects but not in SLE. Our results demonstrate that normal censoring of 9G4 cells is achieved through a combination of mechanisms including germinal center (GC) exclusion, anergy and positive selection into the marginal zone compartment due at least in part to receptor editing. We have also demonstrated that SLE 9G4 cells are expanded in newly identified effector B cell populations in patients with active disease, presumably reflecting the malfunction of GC and late checkpoints. This application will provide a detailed understanding of human B cell tolerance and its breakdown in SLE by pursuing the following Aims: 1) To elucidate the role of receptor editing and antigen selection in human B cell tolerance;2) To understand antigen-driven selection of effector autoimmune B cells in SLE;and 3) To understand the role of regulatory T cells in human B cell tolerance. The expected information will substantially advance the existing knowledge while also identifying B cell markers and autoimmune specificities with great potential as biomarkers, diagnostic and therapeutic targets.